Each 837p batch row contributes up to two entries to the join-key
index (envelope.control_number + transaction_set_control_number). One
idx.get(set_control_number) call resolves either, so 999 acks whose
AK201 echoes the source 837's ST02 now hit Pass 1 where they
previously fell through to Pass 2 (and to the orphan log).
Backward compat preserved: rows with transaction_set_control_number
NULL (pre-SP37 batches) still resolve by ISA13.
Nora